The present invention relates to devices for dispensing measured amounts of mercury and sorbing certain gases. More particularly, the present invention relates to devices for dispensing mercury, sorbing reactive gases, shielding electrodes in fluorescent lamps, and processes for making such devices.
A fluorescent lamp typically includes a glass tube which may be either rectilinear or circular depending on the type of lamp employed. The inner surface of the glass tube is generally lined with powders of fluorescent materials, known as phosphors, which are responsible for the emission of visible light when activated. The tube is typically filled with a rare gas, such as argon or neon, including small quantities of mercury vapors, i.e., quantities on the order of a few milligrams (mg). Two electrodes functioning as cathodes are formed inside the tube by placing metal wires, for example, at both ends of the tube in the rectilinear lamp or in a given zone in the circular lamp.
When the lamp is energized, a potential difference between the two electrodes generates an electronic emission and strikes a plasma inside the tube. It is believed that the plasma contains free electrons and ions of the rare gas, which propel the mercury atoms to a higher excitation state and cause the emission of UV radiation. The phosphors absorb the UV radiation emitted by the mercury atoms and through the fluorescence phenomenon emit visible light. Mercury, therefore is an integral component in the effective operation of the lamp.
Mercury is typically provided in the lamp in a minimum quantity, below which the lamp does not work. It is undesirable to employ mercury in quantities greater than the necessary minimum as the disposal of toxic mercury at the end of the life of the lamp, e.g., due to breakage, etc., poses serious health and environmental problems. Thus, it is important to provide mercury inside the lamp in extremely precise quantities and a reproducible manner. However, this may be particularly complicated because the variety of lamps appearing on the market, having different shapes, sizes and component materials, has significantly increased and the quantity of mercury required for lamp operation varies from lamp to lamp.
Conventional methods of providing mercury in its liquid state has not proved to be reliable, due in part to the difficulty in providing precise and reproducible volumes of liquid mercury in the range of a few microliters (.mu.l) and the uncontrolled diffusion of mercury vapors in the working area. As a result, various other methods have been proposed as alternatives to the conventional method of providing liquid mercury.
One such alternative includes the use of amalgams containing elements such as zinc to provide mercury in the lamps during the lamp assembly process. However, these amalgams tend to release mercury at the relatively low temperatures of about 100.degree. C. The release of mercury becomes especially serious during the lamp manufacturing process when the lamp is open and exposed to high temperatures, as mercury is then released into the manufacturing environment posing health and contamination threats to those in the production area.
Another alternative to the conventional method of providing mercury includes the use of capsules containing liquid mercury as suggested by U.S. Pat. Nos. 4,823,047 and 4,754,193. This method of providing mercury, however, is also unreliable for similar reasons described above. Furthermore, it is also difficult to manufacture capsules in small sizes that are necessary for many lamp designs. The alternative use of pellets or pills of porous materials soaked with liquid mercury, as suggested by U.S. Pat. Nos. 4,808,136 and EP-A-568317, has also not been found to be an effective method for providing mercury in the lamp because the positioning of the pellets in the lamp is an extremely arduous and a time-consuming task.
As a further example of an alternative method of providing mercury in the lamp, U.S. Pat. No. 3,657,589 discloses the use of intermetallic compounds of mercury with titanium and/or zirconium for providing precise quantities of mercury in lamps. The intermetallic compounds are well suited for providing mercury because they are stable at high temperatures, e.g., about 500.degree. C., generally encountered during the manufacturing process of the lamps. One such material, Ti.sub.3 Hg, is commercially available from SAES GETTERS S.p.A. of Lainate (Milano), Italy, under the tradename St 505. According to U.S. Pat. No. 3,657,589, the St 505 compound can be introduced into the lamp both in free form, such as compressed powders, or in supported form, e.g., as powder pressed on an open container or supported on a metallic strip. The supported form is particularly appreciated by the manufacturers of lamps because the strip carrying the mercury dispensing material can be closed as a ring, which simultaneously functions as an electrode shielding member.
After the lamp is assembled and sealed, the St 505 compound typically undergoes an activation treatment step, which includes heating the compound by radio frequency (RF) waves produced by an external coil for about 30 seconds at temperatures of about 900.degree. C., to release mercury. The mercury yield of these compounds during activation is less than 50% and the remaining mercury is slowly released during the life of the lamp. European Patent Application Nos. 95830046.9 (EP-A-0669639) and 95830284.6 (EP-A-0691670) suggest mixing the above-mentioned mercury intermetallic compounds with promoting alloys, such as copper-tin and copper-silicon alloys. The promoting alloys facilitate the release of mercury from the intermetallic compound during the activation step, and thereby shorten heating times or lower temperatures during activation.
The operation of a fluorescent lamp is also significantly impaired by the presence of reactive gases inside the lamp. By way of example, hydrogen (H.sub.2) interacts with a fraction of the electrons emitted during the decomposition of the rare gas and thereby increases the minimum voltage required to switch on the lamp. Other examples of reactive gases that impair the lamp's operation include: oxygen (O.sub.2) and water (H.sub.2 O), which undesirably remove mercury by producing mercury oxide; and carbon oxides, such as carbon monoxide (CO) and carbon dioxide (CO.sub.2), which decompose when they come in contact with the electrodes to form oxygen (O.sub.2), (which removes mercury as mentioned above) and carbon (C), which deposits on the phosphors to create dark zones in the lamp. In order for the lamps to function effectively, it is important, therefore, to remove such reactive gases by providing means for sorbing reactive gases inside the lamps.
To this end, EP-A-0669639 and EP-A-0691670 suggest adding powders of a getter material to the powders of the mercury releasing material to facilitate the sorption of the above-mentioned reactive gases. The getter material most commonly employed is an alloy having percent composition by weight of 84% Zr, 16% Al, available commercially from SAES GETTERS S.p.A. of Lainate (Milano), Italy, under the tradename St 101. Other suitable getter alloys include alloys having the following percent compositions by weight: 70% Zr; 24.6% V; 5.4% Fe and 76.6% Zr; 23.4% Fe, also available from SAES GETTERS S.p.A. of Lainate (Milano), Italy, under the tradename St 707 and St 198, respectively.
To this end, a "shield" including metallic support strips placed co-axially in the lamp, is also provided to prevent blackening of the phosphors in the electrode areas. The shield includes both the getter material and the mercury releasing material deposited directly on the shielding members surrounding the electrodes. One such shield configuration is described in U.S. Pat. No. 3,657,589.
However, when the above-described copper-based promoting alloys are employed with a shield as described above, it is not possible to mix the getter material with the mercury releasing material as the copper-based alloys melt and at least partially coat the getter surface at temperatures required to activate the release of mercury from the mercury releasing material. Consequently, this impedes the ability of the getter to effectively sorb reactive gases. It is, therefore, preferable to keep the getter material separated from the mercury releasing material when promoting alloys are employed in the lamp. This can be accomplished by depositing separate tracks of powdered mercury releasing material and powdered getter on a strip-shaped support. In this context, the above-mentioned European patent applications suggest the possibility of depositing the two powders on the opposite sides of the support strip by cold rolling. According to this technique, the cold support strip and powders are positioned appropriately and passed through pressure rollers to form tracks of powder on the opposite sides of the same strip.
Unfortunately, this process suffers from several drawbacks. By way of example, it is difficult in practice to carry out the deposition on the opposite sides of the support strip. In particular, it is difficult to pass the support strip vertically between two rollers positioned on the opposite sides of the support strip, while pouring two different powders on the opposite sides in a single working step. There is also a potential risk that the first deposit track may be removed or somehow altered during a second rolling step when the deposition is being carried out on the opposite sides in two distinct passages. As the strip is bent to produce a shield, there is a potential risk of removing the powder deposition from the strip, particularly from the concave region of the bent strip. Furthermore, rolling different powders having different hardness induce mechanical strains of varying intensities, which if not balanced may ultimately deform the strip, e.g., the strip may stretch along one of its sides, resulting in lateral bending or "sabre-blade" shaping.
Thus, it would be advantageous to provide a mercury dispensing device that can effectively sorb reactive gases and shield electrodes in a fluorescent lamp without suffering from the prior art drawbacks, e.g., poor getter performance and lateral bending or "sabre-blade" shaping.